The 3rd Generation Partnership Project (3GPP) is responsible for the standardization of the Universal Mobile Telecommunication System (UMTS) and Long Term Evolution (LTE). The 3GPP work on LTE is also referred to as Evolved Universal Terrestrial Access Network (E-UTRAN). LTE is a technology for realizing high-speed packet-based communication that can reach high data rates both in the downlink and in the uplink, and is thought of as a next generation mobile communication system relative to UMTS.
In LTE, OFDM (Orthogonal Frequency Division Multiplexing) is used in the downlink. The LTE physical resource can be seen as a time-frequency grid, where each resource element, i.e. each square in the grid, corresponds to one OFDM subcarrier during one OFDM symbol interval. An LTE downlink subframe comprising 14 OFDM symbols, with 3 OFDM symbols as control region, is illustrated in FIG. 2a. The control region comprises e.g. the Physical Downlink Control Channel (PDCCH), on which control information such as downlink scheduling assignments and uplink scheduling grants are transmitted. In the data region, data is transmitted on the Physical Downlink Shared Channel (PDSCH). Some of the resource elements within the time-frequency grid are used to transmit reference symbols (RS), which are known symbols which may e.g. be used by the receiver for channel estimation in order to perform coherent demodulation.
There is an ever increasing demand for higher data rates in cellular networks, which poses challenges to developers of such wireless networks. One approach to meeting requirements for higher data rates is to deploy heterogeneous networks, i.e. a network containing base stations with different transmission power. Base stations operating with high transmission power are herein denoted macro base stations, and base stations operating with lower transmission power are denoted low power nodes (LPN), but may also be referred to by other terms such as micro, pico, or femto base stations. The LPNs may further be stand-alone base stations, relays, or remote radio units.
Cell selection by wireless terminals is typically based on downlink (DL) received power, including the effects of the different base station transmission power. This leads to an ‘imbalance area’ surrounding the low power node where the path loss is lower towards the low power node, but the macro base station is still selected due to its higher transmission power. In the uplink (UL) direction, where the transmit power is the same, it would be better for a wireless terminal to be connected to the low power node also in this area. By increasing transmission power of the lower power node, the cell size of low power nodes can be increased. However, doing so affects the cost and size of the node, which in turn limits site availability. The range of the low power node can also be increased using a cell selection offset that favors the selection of the low power node. This leads to the UL being received in the best node, i.e. the low power node, and offloads the macro to a greater extent. These benefits, however, come at the cost of higher DL interference for users on the edge of the low power node cell. To mitigate this interference, the interfering macro base stations can be silenced. This is supported in LTE release 10 through so called Almost Blank Subframes (ABS). 3GPP Technical Report 36.300, version 10.3.0, section 16.1.5 defines ABS in the following way: “Almost blank subframes are subframes with reduced transmit power (including no transmission) on some physical channels and/or reduced activity. The eNB ensures backwards compatibility towards UEs by transmitting necessary control channels and physical signals as well as System Information.”